Brain tumors cause more deaths in children than any other form of cancer. Most pediatric brain tumor patients receive surgery and radiation as key elements of treatment. To help surgeons maximally and safely remove brain tumors, we previously discovered and developed Tumor Paint, which delivers fluorescent signal to brain tumor cells in pediatric clinical trials. Chlorotoxin (CTX), the scorpion-derived tumor targeting peptide, crosses the blood brain barrier (BBB) and specifically binds to cancer cells. Because chlorotoxin can deliver fluorescent molecules to the cytoplasm of brain tumor cells, we hypothesized that it could carry therapeutic molecules as well. As we focus on developing therapeutic candidates that use CTX or CTX pharmacophores, it becomes essential to understand the mechanism of BBB penetration. In addition to work on CTX-based brain tumor therapies (e.g., delivery of chemotherapy or immunotherapy to brain tumors), we have made significant progress on a candidate drug that could potentially help every child who undergoes radiation therapy for brain tumors. Because brain irradiation causes severe and irreversible neurocognitive damage in children, we aspire to engineer a therapeutic agent that blocks the toxic respiratory burst of microglia in normal brain following radiation. Blockade of the Kv1.3 potassium ion channel on microglia has been shown to block radiation damage to normal brain in mice. We have engineered an optide (optimized peptide) that specifically blocks Kv1.3 but unfortunately does not, in its current form, cross the BBB. The gap in knowledge that we intend to address is that the mechanism by which CTX and some other optides penetrate the BBB is unknown. Because the Lys27 face of CTX is sterically hindered by a fluorophore in the Tumor Paint clinical candidate that crosses the BBB in children, we hypothesize that the pharmacophore responsible for BBB penetration lies on a different face than the face that contains Lys27. The key hurdle that prevents clinical development of an optide that blocks Kv1.3 to alleviate radiation-induced brain damage is that it does not cross the BBB and therefore fails to reach its target. We hypothesize that we can engineer the candidate Kv1.3 blocker in a manner that fosters BBB penetration. Our Specific Aims are: Aim 1: To identify the pharmacophore of chlorotoxin responsible for BBB penetration Aim 2: To identify the transporter responsible for optide penetration of the BBB Aim 3: To create an optide that has a therapeutic pharmacophore and a BBB-penetrating pharmacophore The significance of this work is that we will produce a clinical development candidate that could alleviate severe brain damage caused by irradiation in children. The foundational knowledge could be applied to a new generation of drugs for many brain disorders.